is a collection effect that defines one or more variables as a single effect with multiple degrees of freedom. The variables
in a collection are considered as a unit for estimation and inference.

LAG

is a classification effect in which the level that is used for a given period corresponds to the level in the preceding period.

MULTIMEMBER | MM

is a multimember classification effect whose levels are determined by one or more variables that appear in a CLASS statement.

POLYNOMIAL | POLY

is a multivariate polynomial effect in the specified numeric variables.

SPLINE

is a regression spline effect whose columns are univariate spline expansions of one or more variables. A spline expansion
replaces the original variable with an expanded or larger set of new variables.